Why is _id field mandatory ? Is it mandatory for every record? Also can it be system generated ?

Hiya Aditi! "_id " is always mandatory. Its a primary key for every document in Mongo. They're like identification and unique and makes your life easier especially when you query the database and hey its considered best practice as well. But there is one gotcha in Mongo. Attempting to use an "id"(not to mistaken with "_id") key may result in a illegal ObjectId format error so yeah look out for those. Also , "_id" is generated with timestamps , so yeah its easier to keep track of Mongo documents. In my opinion its pretty awesome that " _id" is automatically indexed in every collection.

Aditi-

As Ari Misha mentioned, each document in a MongoDB collection has an _id. By default _id is assigned an ObjectId value which includes a lot of different information. Part of that information, the first four of the twelve bits, is a timestamp. Based on the specifications for MongoDB, ObjectId's are guaranteed to be unique in a single collection, but not necessarily across collections.

However, one can also generate and assign values to the _id field. There are definitely some use cases for that. However, most of the time I find it best to let MongoDB generate the _id field and then if it is necessary for the application to have another per document unique field, create that separately. Have a look here for an example of some reasons that not using an ObjectId might be a good idea.

As with most database schema designs, how you plan on using your data will have a large impact on the "best" method for storing the data.

Post back if you have further questions or want to discuss MongoDB in further detail.

Happy coding! Ken